1. Technical Field
This invention relates generally to the field of gas dryers. In particular, this invention relates to a desiccant gas dryer of the kind having a pair of sorbent or desiccant beds, wherein one bed is regenerated as the other bed adsorbs vapor.
2. Description of the Related Art
The desirability of drying air after compression is well known. A variety of systems have been developed over the years for this purpose, many of which employ two beds containing an adsorbent or desiccant material such as activated alumina. In such systems the beds alternately dry the process stream and then are regenerated either using an auxiliary source of heat in so-called heat-reactivated systems, or through conservation and use of the heat of adsorption for effecting regeneration, in so-called heaterless pressure-swing systems. In heat-reactivated systems, it has been common to use fixed cycles several hours in length, such that a bed is on adsorption for a period of time, perhaps eight hours, after which it is regenerated during the eight hour adsorption cycle for the other bed. In the heaterless systems relatively short, fixed cycle times are used, typically about four minutes for drying with four minutes allowed for regeneration while the other bed is drying the process stream.
Typically both heat-reactivated and heaterless systems are sized for maximum contemplated loading conditions, conditions which may seldom or never be encountered during actual operation. As a result, at the end of a fixed drying cycle a bed may have adsorbed only a fraction of its moisture capacity. In the case of heat-reactivated dryers, such underutilization of the bed while on adsorption coupled with a full energy regeneration can seriously affect overall operating efficiency.
Similarly, off-peak operation of heaterless dryers on fixed cycles can result in low operation efficiencies. By their nature heaterless dryers employ approximately 15% of the dry product air for regeneration purposes such that unnecessarily purging a partially loaded bed for the full regeneration period is highly inefficient.
Over the years "demand" rather than fixed cycling has been employed in both heat-reactivated and heaterless dryers. Seibert, et al. U.S. Pat. No. 3,448,561 discloses a system for sensing the actual moisture content of the on-stream bed and terminating the adsorption cycle when regeneration is complete. Alternatively, the patent suggests that the moisture content of the regenerated bed can be monitored, and the purge flow can be terminated when the moisture content of the bed has dropped below a predetermined minimum. That patent discloses the use of a lithium chloride humidity sensor external to the desiccant beds with gas sampling conduit leading from the beds to the cell. This method, however, was not found to be commercially successful for demand control of heaterless dryers, due to the relatively short cycle times in these heaterless systems.
A brute-force technique for demand control of heaterless dryers is disclosed by White, Jr. et al. U.S. Pat. No. 4,197,095. White, Jr. et al. teach that the flow rate, inlet and outlet temperatures, inlet and outlet pressures, and regenerating pressures could be sensed, and all the sensed information should be fed to a microprocessor programmed to calculate the quantity of purge and the purge flow rate, and based on these calculations, control the regeneration and cycling time. Such a control method, however, has had limited commercial application due to the relatively high cost of reliable sensors and their associated electronics.
To date, considerable success has been achieved in sensing the moisture loading on a desiccant bed by placing a capacitor directly into the bed with the sorbent or desiccant between the capacitor plates. As the moisture loading varies, the dielectric constant of the sorbent changes, such that the capacitance provides an electrical indication of the moisture loading. The use of such a capacitor probe for the control of a heaterless dryer is disclosed in Gravatt U.S. Pat. No. 4,552,570 issued Nov. 12, 1985. The capacitor probes are placed in the middle of the desiccant beds, and a "fixed-variable" control method is used in which the adsorption and cycle times are fixed but in which the off-stream bed is conditionally purged and regenerated based upon whether, at the end of each adsorption interval, the capacitor in the respective bed having undergone adsorption indicates that regeneration is required. This "fixed-variable" control method tends to equalize loading on the beds by preventing the capacity of the heavier-loaded bed from being reduced in the event of a sudden increase in loading.
A microcomputer-based control system for a desiccant dryer employing the "fixed-variable" control method is described in Tinker U.S. Pat. No. 4,546,422 issued Oct. 8, 1985 which is herein incorporated by reference.